The present invention relates to an arrangement adapted to provide in ultrasonic sealing by means of a sealing jaw and a counterjaw a working gap of predetermined dimension between the working surfaces of the jaws.
In the packaging industry thermoplastic material is frequently used for the manufacture of packing containers of the non-returnable type, e.g. packages for milk or other liquid dairy products. The packing material is in the form of sheets or webs and usually comprises several layers, e.g. a central carrier layer of fibrous material, e.g. paper, imparting rigidity and layers of liquid-tight, thermoplastic material applied to either side of it. The liquid-absorbing properties of the fibrous carrier layer can be a disadvantage in certain cases, and this layer is sometimes replaced therefore in modern packing laminates by a layer of foamed thermoplastic material which serves as a spacer between the two homogeneous surface layers and thereby imparts great rigidity to the packing material in relation to the total weight.
The conversion of the packing laminate to liquid-filled individual packing containers usually takes place in steps in that one or more material webs are converted by means of different types or processing, e.g. cutting, folding, thermoforming and sealing to a material tube consisting of coherent packing container blanks. During the forming work the material tube preferably moves more or less vertically downwards through the packing machine and is filled in the process continuously with contents to a predetermined level. At the same time a transverse sealing, repeated at equal intervals, takes place at the lower end of the material tube, which divides the tube into individual, liquid-tight packing containers. The packing containers are severed from one another by cutting through the transverse sealing zones and after possible further form-processing or other treatment they are ready to be transported away to storage or sales points.
During the forming work, the material web as mentioned earlier, is sealed on several occasions to itself or to other material webs. The demands on these seals are great both in respect of strength and of liquid-tightness and it has proved appropriate, therefore, to carry them out with the help of heat sealing. In this process the thermoplastic packing laminate layers are heated with simultaneous or subsequent compression to such a temperature that they soften or fuse together thus giving a homogeneous seal with good properties, well suited for the purpose. The heat sealing can be realized either with the help of heat generated outside the packing laminate, e.g. through convection from surrounding heat sources, or through heat which is generated directly in the packing laminate, e.g. through microwave energy. The latter method is used to an increasing extent and has been found to be particularly appropriate to be used when the transverse seals mentioned earlier are to be accomplished. In this sealing work the laminate layers forming the packing material tube must be compressed first whilst displacing at the same time any contents present between them, whereupon the thermoplastic layers in the sealing zone can be united by heating them to melting temperature. Especially in the manufacture of larger types of packing containers with a volume of 1 liter or more the material tube must be of such a size that the length of the transverse seal will be realtively great, e.g. 200 mm. At the same time the seal width amounts to not more than a few millimetres. The great length of the seal creates problems in the sealing work, since even a very slight angular deviation of one of the sealing jaws between which the material is compressed during the sealing work brings about an uneven sealing quality which appreciably increases the danger of leakage. These difficulties moreover clearly increase in ultrasonic sealing of wholly plastic laminate, that is to say packing laminate which lacks a carrier layer of fibrous material. This type of packing laminate instead has a central layer of foamed plastic material which in heated condition offers little or no resistance when the laminate layers are pressed together by the sealing jaws. In ultrasonic sealing the jaws are pressed against each other continuously during the sealing process with a predetermined force whilst the energy transmission is controlled. Since the energy transmission in ultrasonic sealing is directly proportional to the sealing pressure any possible angular deviation of the sealing jaw means that the sealing pressure will be unevenly distributed over the length of the sealing jaw, with all of the ultrasonic energy being conducted to that part of the sealing region where the gap between the sealing jaws is smallest. This means, though, that the material is fully burnt through in this region while no sealing whatever takes place at the opposite end of the sealing jaw. In order to overcome these disadvantages up to now in that very small manufacturing tolerances and high demands on accuracy have been specified in respect of sealing jaws and associated elements. The maximum permissible deviation in parallelism between the working surfaces of the jaws e.g. is 0.05 mm which is extraordinarily small in a jaw length of up to 200 mm. Obviously this gives rise to difficulties as well as to high costs in the manufacture of the parts concerned and during a prolonged period in practical use there is reason to fear, moreover, that the demand for accuracy cannot be maintained because of the wear occurring, which evidently results in varying sealing quality and a high requirement of service.
It is generally desirable therefore to provide an arrangement for ultrasonic sealing, this arrangement being not sensitive to angular deviations between the jaws or thickness variations of the material layers which are to be sealed together.
lt is an object of the present invention, therefore, to provide such an arrangement which is particularly suitable for ultrasonic sealing in the packaging industry where comparatively thin theremoplastic material is often provided with relatively long seals and where the demands on the quality and liquid-tightness of the seal are extremely high.
lt is a further object of the present invention to provide an arrangement for ultrasonic sealing, which is not subject to the disadvantages of earlier known similar arrangements and which is capable of being manufactured in an uncomplicated manner at reasonable costs.
It is a further object of the present invention finally to provide an arrangement for ultrasonic sealing, which is not very complicated and well adapted for use in modern machines for the manufacture of liquid-filled packing containers.
These and other objects have been achieved in accordance with the invention in that an arrangement adapted to provide in ultrasonic sealing, by means of a sealing jaw and a counterjaw, a working gap of predetermined dimension located between the working surfaces of the jaws has been given the characteristic that supporting elements are arranged to determine the minimum distance between the working surfaces of the jaws, these supporting elements in the working position of the working jaw resting against a stop face formed in the nodal plane of the working jaw.
By making use of supporting elements which limit the working movement and define the gap width, a uniform heating and a seal of good quality are obtained without the supply of energy having to be monitored or controlled which means that an appreciable simplification of the sealing equipment is made possible. The direct contact of the supporting elements against the working jaw in practice eliminates the risk of any play or flexibilities which are present or arise in the sealing equipment as well as in the surrounding elements and which negatively affect the gap width, critical for the sealing result, between the working surfaces of the jaws. The use of two or more stop faces on the sealing jaw and the flexible suspension of the counterjaw make it possible fully to disengage the normal driving and control arrangement of the jaws during the important end phase of the sealing work proper which means that inaccuracies in these components wholly lack the opportunity of influencing the sealing result.
The arrangement in accordance with the invention, on being used in packing machines for the manufacture of liquid-filled non-returnable packages has proved to bring about such an improvement that it has become possible only now to make use of ultrasonic sealing with sufficient operational safety and accuracy on greater sealing lengths.